This letter presents a new transmitter for multiband impulse radio ultra-wideband (IR-UWB) systems. The ultra low-power, low-complexity UWB transmitter operates over three 528-MHz subbands in 3-5 GHz band. It consists of an On-Off Keying (OOK) modulator and a pulse generator which is based on the ON/OFF switching operation of an LC oscillator. Measurements show a pulse duration of 3.5 ns and a spectrum that fully complies with the FCC spectral mask with more than 20 dB of sidelobe rejection. Implemented in 0.18-mu m CMOS technology, the transmitter operates in burst mode and dissipates only 18 pJ of energy consumption per pulse. The transmitter is best suited for energy detection receivers.